The Hall-Heroult process is a well-know method used for mass-producing aluminum (which metal is also sometimes referred to as “aluminium”). This process uses electrolytic cells in which purified alumina is dissolved into a mixture having a large content of molten cryolite. The electrodes used in a Hall-Heroult cell are generally made of a carbonaceous material having a good electrical conductivity. The cathode is a permanent electrode that can last many years and at least one is placed at the bottom of a cell. Each cell generally contains a multitude of anodes placed at the top thereof. Aluminum is produced when a large electric current go through the electrodes. Under the influence of the current, the oxygen of the alumina is deposited on the anodes and is released as carbon dioxide, while free molten aluminum, which is heavier than the electrolyte, is deposited on the cathode at the bottom of the cell. The anodes are thus not permanent and are consumed according to the aluminum production rate. They must be replaced once they have reached their useful life.
A large part of the world production of aluminum is obtained from Hall-Heroult cells that use pre-baked anodes. Pre-baked anodes are consumed in about 10 to 45 days. A typical large Hall-Heroult cell can contain more than twenty anodes. Since an aluminum smelter can have many hundreds of cells in a single plant, it is therefore necessary to produce and replace each day several hundreds of anodes. Having an adequate supply of good anodes is a major concern for aluminum smelters.
Anodes are usually made from two basic materials, namely petroleum coke and pitch. Coke is a solid material that must be heated at a high temperature before use. Pitch is a viscous and sticky material that binds solid particles of coke together and increases the surface of contact between particles. Having a larger surface of contact between particles increases the electrical conductivity of the anodes. However, adding a too high proportion of pitch usually creates porosities that decrease the electrical conductivity of the anodes. There is thus an optimum proportion of pitch in the composition of the crude anodes. Typically, the mixture contains between 10 and 20% by weight of pitch, which generally yields a product having a good cohesion and an adequate electrical conductivity.
Optimizing the electrical conductivity of anodes is relatively important in terms of operation costs. When the current flows through the anodes, a part of the energy is transformed into heat. This energy is wasted and must be minimized to improve the efficiency of the process and the aluminum production rate. Therefore, anodes must ideally have the highest possible electrical conductivity.
The percentage of pitch is generally adjusted according to the size distribution of coke particles. Higher content of pitch is necessary to bind particle of smaller diameter. When the target composition of the mixture is obtained, a pre-defined amount is pressed and possibly vibrated into a mold having the form of the anode. The resulting product coming out of the mold is a crude anode block weighing between 500 to 1500 kg. Then, the crude anode must be baked, typically for 10 to 15 days, to decompose the pitch into carbon so as to create a permanent binding between coke particles. The baking of anodes is usually done in pits in which a large number of anodes is set. It only after the baking that the electrical conductivity of the anodes can be measured using conventional measuring devices. Before baking, any measurements using these conventional devices are generally unreliable. The electrical conductivity of baked anodes can also be measured when they are in operation in a cell.
As can be seen, any unintentional variation occurring during the manufacturing process of the anodes may go undetected until the baking of these anodes is completed, thus many days after their manufacturing process started. Many factors can affect the electrical conductivity of anodes, all of which represent challenges for the manufacturers of anodes. One of these challenges is the variation of the coke particle size. Typically, coke particle size can vary from 100 microns to 5 cm. The size distribution can vary from one batch to another, thereby resulting in anodes of different electrical conductivity unless the pitch proportion is adjusted accordingly. Another challenge is to keep an accurate proportion of ingredients in the mixture, particularly the pitch. Pitch is a highly viscous product difficult to handle so that the exact amount supplied by the pitch distribution apparatus to the initial mixture may vary from one batch to another. There are also other challenges, such as obtaining a very homogenous mixture of the ingredients, preventing air from being entrapped in the mixture and create voids, obtaining an optimal compaction of the mixture in the molds before baking, and preventing elastic deformation of the coke particles in effort to avoid layer separation in the blocks. All these factors may potentially shift the electrical conductivity of one or several anodes out of the target value. As aforesaid, this will only be known once the anodes are baked, thus many days later. At that point, corrections can be made to the manufacturing process but the anodes already manufactured or currently being baked may be defective or otherwise less desirable.